Transcript Severe Weather Guide

Severe Weather Guide
How to Recognize, Identify, and
Report Severe Weather
Definitions and Terms
• Watch: conditions are favorable for severe
weather
• Warning: severe weather is currently
occurring in the area
• Watches and warnings are issued for:
severe thunderstorms, hail, flash floods,
and tornadoes
Thunderstorms
• Every Thunderstorm Needs:
– Moisture
– Unstable Air (warm air to rise rapidly)
– Lift (cold front)
• There are three stages in the life cycle of a
thunderstorm
– Developing (Cumulus) stage
– Mature stage
– Dissipating stage
Thunderstorms
• Developing Stage
– Rising cumulus
clouds; strong updraft
– Little if any
precipitation
– Lasts about 10
minutes
– Occasional lightning
Thunderstorms
• Mature Stage
– Updraft and downdraft
– Most likely time for
heavy rain, frequent
lightning, strong winds,
and hail
– Tornado development
is possible
– Averages 10-20
minutes, but can last a
few hours
Thunderstorms
• Dissipating Stage
– Weakened updraft
– Rainfall lessens in
intensity
– Lightning and strong
winds remain a threat
Types of Thunderstorms
• Single Cell (Pulse):
– Generally weak, short lived, and poorly organized
• Multicell Cluster:
– Most common type
– Series of cells moving as one unit
• Multicell Line:
– AKA “Squall Line”
– Long line of storms with gust front at leading edge
• Supercell:
– Very strong and produce severe weather
Types of Thunderstorms
• A thunderstorm is classified as severe if it
has any of the following characteristics
– Hail greater than 0.75” in diameter (dime size)
– Winds greater than 58 miles per hour
– Tornado
Single Cell Storm
• 20-30 minutes
• Rarely turn
severe
• Heavy rainfall
and weak
tornadoes are still
possible
• Poorly organized
Single Cell Storm
Multicell Cluster Storm
• Most common type of thunderstorm
• Each cell in the cluster is at a different
stage of the thunderstorm life cycle
• Each cell may last 20 minutes, but each
cluster can last several hours
• Heavy rain, downbursts, moderate sized
hail, occasional weak tornadoes
Multicell Cluster Storm
Multicell Cluster Storm
Multicell Line Storm
• Squall Line
• Long line of storms with a continuous, well
developed gust front at leading edge of the
line
• Heaviest rain is at center of line
• Produce heavy rain, hail, and tornadoes
• Strong downbursts can cause line to bend
and become a “bow echo”
Multicell Line Storm
Multicell Line Storm
Multicell Line Storm – Bow Echo
Multicell Line Storm – Bow Echo
Supercell Thunderstorm
• Rarest type of thunderstorm, but the most
dangerous
• The updraft rotates (called mesocyclone)
• Large hail
• Heavy downpours
• Strong downbursts
• Strong to violent tornadoes
Supercell Thunderstorm (w/o Tornado)
Supercell Thunderstorm (w/Tornado)
Supercell Thunderstorm (w/Tornado)
Supercell Thunderstorm
Dangers of Thunderstorms
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Flash Floods
Lightning
Hail
Downbursts
Tornadoes
Flash Floods
• #1 cause of death associated with
thunderstorms
• An average of 140 fatalities every year (in US)
• Definition: a rapid rise in water (creeks, streams,
drainage ditches) within 12 hours of a period of
heavy rain
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As little as 6 inches can knock a human over
Two feet of water can move a car
“Turn Around, Don’t Drown”
Get to higher ground immediately
Lightning
• Lightning occurs in all thunderstorms
• Causes an average of 80 fatalities and 300
injuries per year (in the US)
• Lightning strikes the tallest object
– If caught outside crouch down in a ball
• 30/30 Rule
– Go indoors if you hear thunder before counting to 30
after you see lightning
– Wait inside for 30 minutes after you last hear thunder
Hail
• Rarely causes
fatalities, but causes
significant damage to
property and crops
• Can fall at rates up to
100 miles per hour
• Created by strong
updrafts in
thunderstorm
Hail
• Sizing Chart
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Pea
Penny/Dime (Severe Criteria)
Nickel
Quarter
Half Dollar
Ping Pong Ball
Golf ball
Hen Egg
Tennis Ball
Baseball
Grapefruit
Softball
0.25”
0.75”
0.88”
1.00”
1.25”
1.50”
1.75”
2.00”
2.50”
2.75”
4.00”
4.50”
Downbursts
• A strong downdraft with an outrush of
damaging winds at the surface
• Winds can reach 100 miles per hour or
more
• Straight line winds
• Winds speed and direction can change
rapidly
Downbursts
Downbursts
• Wind Speed Estimates (mph)
– 25-31: large branches in motion; whistling in
telephone wires
– 32-38: whole trees in motion
– 39-54: twigs break off of trees; wind impedes walking
– 55-72: damage to chimneys and TV antennas;
pushes over shallow rooted trees
– 73-112: peels surface off roofs; windows broken;
trailer houses overturned
– 113+: roofs torn off houses; weak building destroyed;
large trees uprooted
Tornado “Look-A-Likes”
• Several cloud formations are associated with a
thunderstorm that can be confused with an
actual tornado
– Wall clouds
– Shelf clouds
– Roll clouds
• A roll cloud is similar to a shelf cloud, but it is detached from
the main “parent cloud” whereas a shelf cloud is part of the
main storm cloud
– Scud Clouds
• Detached and wind torn – similar shape to wall/funnel clouds
Wall Clouds versus Shelf Clouds
Wall Cloud
Shelf Cloud
Suggest Inflow/Updraft
Suggest
Outflow/Downdraft
Maintain position with
respect to rain
Move away from rain
Slope upward away from Slope downward away
precipitation
from precipitation
Wall Cloud
Wall Cloud
• What is the tornado potential for a wall
cloud?
– It will be consistent, lasting 10-20 minutes
– It will have persistent rotation
– Strong winds will blow into the wall cloud from
the south or southeast (25-35 mph)
– It will exhibit evidence of rapid vertical motion
• These are rules of thumb – there are
always exceptions!
Shelf Cloud
Roll Cloud
Scud Cloud
Tornadoes
• A tornado is a violently rotating column of
air in contact with the ground extending
from a thunderstorm
• May appear transparent until dirt and
debris are picked up in the vortex or until a
condensation cloud forms
• A tornado that forms over a body of water
is called a waterspout
Tornadoes
• Tornadoes can occur at any time, any day,
and in any state
• They are most common
– In tornado alley: Texas north to Nebraska and
east to Indiana
– During the spring and summer months
– During the late afternoon and early evening
Average Number of Tornadoes Per Year
Average Number of Tornadoes Per Month
Tornadoes By Hour of Day
Tornado Life Cycle
• 1. Funnel Cloud: extending from wall
cloud, but not yet in contact with the
ground
• 2. Mature Tornado
• 3. Rope Stage: the dissipating stage
• Tornadoes are dangerous during all stages
Funnel Cloud
Mature Stage
Rope Tornado
Tornado Characteristics
Weak
Tornadoes
Strong
Tornadoes
Violent
Tornadoes
% of Tornadoes
88%
11%
<1%
% of Tornado
Deaths
<5%
~30%
70%
Duration
1-10+ minutes
20+ minutes
Can exceed 1
hour
Path Length
Up to 3 miles
15+ miles
50+ miles
Winds
<110 mph
110-205mph
>205mph
Tornadoes
• Tornadoes are always dangerous
regardless of shape, size, or color
• Large cities and mountains are just as
prone to tornadoes as wide open fields
• Average tornado speed is 30mph, but they
can move as fast as 70mph
– Do not attempt to outrun a tornado, find
shelter immediately
Tornado Fujita Scale
F Scale
Type
Strength
Winds
0
Gale
Weak
40-72 mph
1
Moderate
Weak
73-112 mph
2
Significant
Strong
113-157 mph
3
Severe
Strong
158-206 mph
4
Devastating Violent
207-260 mph
5
Incredible
261-318 mph
Violent
Severe Weather Alerts
• During periods of severe weather it is
important to keep an eye on the sky
• Local television, radio, and the Internet are
vital sources of information
• A S.A.M.E NOAA weather radio is
essential for immediate watches and
warnings